Plasticity of connections within the neocortical microcircuit is thought to be the substrate for perceptual learning and memory. Mouse whisker barrel cortex offers many unique experimental advantages for the study of neocortical plasticity because of an advantageous sensory and motor periphery and because of the obviousness and accessibility of its columnar architecture, which is highly similar to the architecture of all other regions of neocortex in rodents and other mammals. The mouse in particular offers special advantages for imaging studies because of its small size and the availability of increasingly powerful genetics tools. It is now widely expected that work on the plasticity of mouse barrel cortex will provide for the next few years one of the most promising avenues toward a general cellular and molecular understanding of neocortical learning and memory mechanisms in health and disease. Array tomography (ATom) (Micheva and Smith 2007) is a groundbreaking new imaging technique developed by the Smith lab, which for the first time allows high-throughput analysis of the molecular architecture of tissue at the single-synapse level. This capacity for high- throughput single-synapse analysis provides an unprecedented opportunity to measure neocortical synaptic changes on a comprehensive and panoramic scale and at a level of detail and quantitative reliability far beyond previous experimental approaches. We will use high- throughput ATom to explore the modification of the synaptic microcircuitry of the mouse whisker barrel by altered sensory stimulation, identify specific synapse populations most subject to such modification and characterize corresponding changes in synapse protein composition and structure. This unique new perspective on changes in the molecular architectures of individual synapses is expected to provide an extraordinarily valuable complement to in vivo studies of whisker barrel plasticity being carried out in many other laboratories. Specific Aim 1: To define the laminar distributions of distinct synaptic populations within a barrel column and assess the variability of this synaptic architecture between columns and animals. Specific Aim 2: To characterize the effect of long term principal whisker stimulation on the laminar distributions of distinct synapse populations within the barrel column microcircuit. Specific Aim 3: To compare the plasticity of distinct populations of synapses onto layer 4 spiny stellate cells and layer 5 pyramidal cells.